In wireless communication systems and terminal equipment, receivers always suffer various interferences of transmit signal leakage from outside and inside, and the interferences have the following influences on duplexers or receiving filters.
First, in a frequency duplexer division (FDD) system, the possibility of out-band interference and transmit signal leakage poses high requirements on the out-band suppression of the duplexer or receiving filter. However, some performance, such as out-band suppression, is always increased at the cost of reducing in-band insertion loss and increasing volume and cost of components. When the out-band suppression of the duplexer or the filter is not high enough, the strong interferences have influence on the linearity of the receiver. For example, in a FDD mobile terminal system having a zero intermediate frequency (ZIF) or very low intermediate frequency (VLIF) architecture, the transmit signal after limited isolation performed by the duplexer will leak into the receiver, such strong transmitter leakage signal increases the requirements on receiver IP2/IP3 greatly, and as a result, in many cases, the receiver needs to add a surface acoustic wave (SAW) filter between the LNA and the mixer to filter out the transmitter leakage, so as to reduce the linearity requirements on the post mixer circuit.
In a first prior art, a method of canceling transmitter leakage by using an anti-phase transmit signal in a duplexer is set forth to improve the suppression of the duplexer on the transmitter leakage. FIG. 1 is a schematic structural view of a duplexer in the prior art. As shown in FIG. 1, according to the solution, coupling of a transmit signal is directly realized in the duplexer, and then the transmit signal is coupled to a receiving port after attenuation and phase shift, such that the transmit signal of the coupling path and the transmitter leakage signal of the main path have the identical amplitudes and reverse phases, and thus the transmit (TX) leakage signal is cancelled, thereby improving the suppression of the duplexer on the TX.
In the first prior art, besides the duplexer, two couplers, one phase shifter, and even one attenuator needs to be added, the components cannot be produced by using the same producing process, and have low integration and high cost. Moreover, the solution has strict requirements on the phase flatness and amplitude flatness of the receiving filter at the transmission frequency band, thus further limiting the actual application scope. Furthermore, the solution can only improve the suppression of the transmitter leakage, but has no effect on other out-band interferences and interferences close to the signal frequencies.
In a second prior art, a method of filtering out out-band interference or part of transmitter leakage by a receiver with an on-chip notch filter or a band-pass filter is set forth. FIG. 2 is a schematic structural view of a “notch filter” in the prior art. As shown in FIG. 2, according to the solution, an RF notch filter is introduced at a cascode stage of a low noise amplifier (LNA) or behind the LNA. According to the solution, for the frequency band with a narrow interval between transmitting and receiving, such as, WCDMA BAND II, the notch filter has limited attenuation on the transmission, which is always no higher than 5 dB, and thus the suppression is very limited. As for out-band interference with a frequency close to the frequency of the received signal, the notch filter substantially has no effect. The notch filter needs to introduce an active circuit part for improving a Q value, but the active circuit will increase the noise of the receiver.
In a third prior art, a method of canceling interference signals or transmitter leakage signals by the receiver with a TX cancellation feedback circuit. FIG. 3 is a schematic structural view of a “TX leakage cancellation” in the prior art. As shown in FIG. 3, according to the solution, at a certain RF node in an RF chip (IC), for example, a node D, a suitable local oscillator (LO) is used, and a MIXI1 and a MIXQ1 down-convert RX(receive) main signals and interference or transmitter leakage signals, a low-pass filter (LPF) selects the interference or transmitter leakage signals after down conversion, and filters out the RX signals, and then a MIXI2 and a MIXQ2 up-converts the interference or transmit signals after being filtered by the LPF, and sends the up-convert result to the same RF node or an input of a component in front of the node thereof through negative feedback, and as shown in FIG. 3, to a SUM2, so as to realize interference cancellation or cancellation of the transmitter leakage signals.
However, according to the solution, a double of frequency mixing is required, the circuit is complex, and the driving load of the LO is heavy. Furthermore, the loop gain of the TX cancellation circuit cannot be high, otherwise large signals are likely saturated, and the cancellation effect is limited. Moreover, the linearity and noise of the cancellation circuit has great noise influence on the receiver.